Fire Detection Fault Enhancement
A fire detection switch for use in a fire detection circuit of a aircraft engine, having: a first resistor and a second resistor disposed in series between a common terminal and an alarm terminal; and a thermally-sensitive element having a different resistance at low temperature than at high temperature, the thermally-sensitive element disposed in series with the second resistor and in parallel with the first resistor.
The application relates to an improved fire detection switch that provides an improved ability to distinguish between a fire condition and a false alarm caused by, for example, a short circuit.
BACKGROUND OF THE ARTConventional fire detection systems in aircraft engines rely on fire detection switches disposed within electrical fire detection circuits. In isolation, such fire detection switches are circuit components connected to common and alarm pins connected to sources of electric current. Such conventional fire detection switches are normally open, so that current does not flow through them in ‘normal’, or operating, conditions, but is directed through a path having a resistor. When a fire detection switch is for example exposed to high temperatures associated with fire, the switch closes to complete the circuit and bypasses the path with a resistor. However, if a short circuit occurs through, for example, damage to the switch, a defect in the switch, or other circuit damage, the same bypassing of the path with a resistor can occur, and may result in a false alarm of fire. Conventional fire detection circuits are not capable of distinguishing between a true fire detection signal and a false signal caused by a short circuit.
SUMMARYThe disclosure herein provides fire detection circuits, and elements and devices for use in fire detection circuits, of engines, including particularly gas turbine or other aircraft engines. A fire detection circuit in accordance with the disclosure can, for example, include: first and second resistors disposed in series between a common terminal and an alarm terminal; and at least one thermally-sensitive element having a different resistance at low temperature than at high temperature, the at least one thermally-sensitive element disposed in series with the second resistor and in parallel with the first resistor.
In various aspects and embodiments, such fire detection devices comprise thermistors composed of electrically resistant materials having higher resistances at low temperatures than at high temperatures. In the same or other embodiments, such fire detection devices can comprise thermally-sensitive switches that change their state (open or closed) in response to temperature changes.
Also provided by the disclosure herein are gas turbine and other engines, including particularly aircraft engines, equipped with fire detection circuits comprising such thermally-sensitive elements.
In order that the disclosure may be readily understood, embodiments of circuits, devices, and elements in accordance with the disclosure are illustrated by way of example in the accompanying drawings.
Further details of the disclosure and advantages of the systems disclosed herein will be apparent from the detailed description included below.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTSCombustible materials found in typical gas turbine engines include fuel, lubricating oil, plastic or rubber seals, and insulation covering electrical wires, for example which are found in many diverse locations throughout the engine.
Fire sensors are usually purchased from specialty manufacturers and may be used to detect fires based heat or temperature conditions, chemical concentrations, particles, or optical changes, and may be installed in any location(s) within the engine that are susceptible to or otherwise suitable for detection of fires. For example, fire detection switches may be mounted in the space 15 between the outside of the engine and the surrounding nacelle, as well as in a space 16 around the fuel manifold outward of the combustor 8 and between the bypass duct 3 and plenum 7.
As will be understood by those skilled in the relevant arts, once they have been made familiar with this disclosure, suitable location(s) for installation of fire detection circuits in accordance with this disclosure will depend upon the configuration of a particular engine, including the location of combustible fluids and other materials in the engine, the manner in which the engine is installed, and materials used in the components of the fire detection system. In the present description, the exact nature of the fire sensor is not relevant, so long as it exhibits resistive qualities which vary with temperature (including, for example, thermistor qualities or the opening and/or closing of a switch).
Fire detection systems can detect faults using resistors in differing circuits to provide a measurable difference in voltage and/or current across the fire detection switch to indicate a fault. Many devices use comparators and logic circuits to detect such voltage differences and provide signals indicating a fault associated with a fire situation. The present description and
As summarized in
A disadvantage of the prior art fire detection circuit is revealed when, for example, a Short Circuit condition exists which includes any damage that results in the fire detection circuit being bypassed. Schematically, for example, a Short Circuit condition could be represented by inserting a conductor (not shown) directly between the common terminal 12 and the alarm terminal 13. Since resistor R1 offers resistance to current flow, most current would bypass the fire detection switch and flow directly through the path of least resistance, i.e. directly between the common terminal 12 and the alarm terminal 13.
As noted above, in the prior art fire detection switch of
Thermally-sensitive element 14 can comprise any one or more devices which exhibit significantly different electrically resistant qualities at low (e.g., engine operating) and high (e.g., fire-related) temperatures. Such devices can, for example, include thermistors and/or temperature-sensitive switches. As will be appreciated by those skilled in the relevant arts, a wide variety of each of these types of thermally-sensitive elements is now known, and doubtless others will hereafter be developed.
Thermistors suitable for use in implementing circuits in accordance with the disclosure include, for example, various types of conductors, including negative temperature coefficient (NTC) semiconductors, which exhibit variation in resistance in response to temperature changes. NTC thermistors may be manufactured, for example, through the use of oxides of iron, nickel, manganese, molybdenum, cobalt, etc. Thermistor elements suitable for use in implementing circuits according to the disclosure are provided by several manufacturers, including Kidde and Meggitt.
Similarly, a wide variety of temperature-sensitive switches are known, many of which are commercially available if forms suitable for implementing systems according to this disclosure.
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Thus a circuit designer is offered a choice of operating and fire-condition circuit preferences. The application describes fire detection devices, in three different physical situations or conditions, as shown in
Although the above description relates to a specific preferred embodiment as presently contemplated by the inventor, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.
Claims
1. A fire detection device for use in a fire detection circuit of a gas turbine engine, the fire detection device comprising:
- a first resistor and a second resistor disposed in series between a common terminal and an alarm terminal; and
- at least one thermally-sensitive element having a different resistance at low temperature than at high temperature, the at least one thermally-sensitive element disposed in series with the second resistor and in parallel with the first resistor.
2. The fire detection device of claim 1, wherein the at least one thermally-sensitive element comprises a thermistor having a higher resistance at low temperature than at high temperature.
3. The fire detection device of claim 1, wherein the at least one thermally-sensitive element comprises a temperature-sensitive switch which is normally open at low temperature, and normally closed at high temperature.
4. The fire detection device of claim 1, wherein the at least one thermally-sensitive element comprises a temperature-sensitive switch which is normally closed at low temperature, and normally open at high temperature.
5. An aircraft engine comprising at least one fire detection circuit, the at least one fire detection circuit comprising:
- a first resistor and a second resistor disposed in series between a common terminal and an alarm terminal; and
- at least one thermally-sensitive element having a different resistance at low temperature than at high temperature, the at least one thermally-sensitive element disposed in series with the second resistor and in parallel with the first resistor.
6. The engine of claim 5, wherein the at least one thermally-sensitive element comprises a thermistor having a higher resistance at low temperature than at high temperature.
7. The engine of claim 5, wherein the at least one thermally-sensitive element comprises a temperature-sensitive switch which is normally open at low temperature, and normally closed at high temperature.
8. The engine of claim 5, wherein the at least one thermally-sensitive element comprises a temperature-sensitive switch which is normally closed at low temperature, and normally open at high temperature.
9. The engine of claim 5, wherein the at least one thermally-sensitive element of the at least one fire detection circuit is located in a portion of the engine susceptible to fire.
Type: Application
Filed: Oct 5, 2009
Publication Date: Apr 7, 2011
Patent Grant number: 8314709
Inventors: Peter Lance (Mississauga), Gordon Ferch (Etobicoke)
Application Number: 12/573,704
International Classification: G08B 17/12 (20060101);